A variety of different embodiments of spacers for locating (i.e., supporting and fixing) fuel rods in position in a nuclear reactor fuel assembly are already known. Fuel rod-locating spacers which are made completely of a zirconium alloy exhibit good mechanical strength while providing a small neutron capture cross-section. To counteract vibration and hence mechanical wear, it is known to provide spring elements as component parts of the structural element of the spacer. However, the resilience of a zirconium alloy is adversely affected by intense nuclear radiation, so that the spring force is reduced over extended exposure times. This deterioration in resilient properties of the spring element is known as "relaxation".
In bimetallic spacers, for example as described in U.S. Pat. Nos. 3,664,924 and 3,674,635, the structural elements are made wholly of a zirconium alloy while the spring elements are constructed of a nickel-based alloy. Each spacer should have a low flow resistance with respect to the coolant which flows through the fuel assembly in the axial direction of the fuel rods. The spring elements should have good resilient properties to ensure secure fixing of the fuel rods. Further, the spacer with its structural elements and spring elements should have little influence on the neutron flux density in the fuel assembly. It may be difficult to find materials and combinations of materials which in a spacer construction provide a low coolant flow resistant, little influence on the neutron flux density while at the same time having good resilient properties. Structural elements made from a zirconium alloy have good mechanical strength and a small neutron capture cross-section and therefore exert little influence on the neutron flux density, but normally they cannot be made sufficiently thin to provide the desirable low coolant flow resistance. Spring elements made from a nickel-based alloy, for example Inconel, have good mechanical strength and, because of their thinness, they do not significantly increase the coolant flow resistance, but because of their relatively large neutron capture cross-section, they do influence the neutron flux density in the spacer. Differences in the irradiation growth of the structural elements and the spring elements must also be taken into consideration.